In computer, information-processing and control systems, it is necessary to store digital data and to retrieve it as desired. In a semiconductor memory, an array of storage or memory cells is used, with each memory cell holding one bit of data. When the information can be randomly put into or taken out of each memory cell or element as required, the array is called a random access memory (RAM) which may be static (SRAM) or dynamic (DRAM). The individual memory cells are addressed by data input and output lines, with each memory cell commonly having two output bit lines for indicating the presence of a "0" or "1" bit read out from the memory cell. The "0" and "1" bits are represented by different voltages which, when stored in the memory cells, may be quite small and may accumulate errors tending to reduce the difference between the respective voltages. Therefore, it is advantageous to include sense amplifiers connected to the output bit lines which are adapted to more accurately detect the voltages appearing on the bit lines and to latch the digital bit indicated thereby to provide a more accurate read out.
One advantageous example of such a sense amplifier includes cross-coupled field effect transistors each having a first current carrying electrode (source or drain electrode) coupled to a respective one of the bit lines and a gate electrode coupled to the other of the bit lines. Second current carrying electrodes (drain or source electrodes) of the transistors are coupled together to receive a control signal which permits or prevents turning ON of the transistors. If, for example, NMOS transistors are used, each transistor will turn ON when the difference between the gate voltage and the source voltage is greater than the threshold voltage of the transistor. The signal to be sensed appears on only one of the bit lines, which then carries a voltage higher or lower than the other bit line, depending on the value of the sensed signal. Consequently, when the control signal applied to the connected second current carrying electrodes is lowered to permit turn ON of the two transistors, the transistor having its gate electrode coupled to the bit line carrying the higher voltage will turn ON first. The other transistor will thereafter be maintained in its off state to latch the information read out from the memory cell.
However, the sensitivity of such a sense amplifier depends critically on the threshold voltage V.sub.th of each field effect transistor. If the threshold voltage of the transistor which is intended to turn ON first becomes significantly larger than the threshold voltage of the other transistor, it may happen that the other transistor will turn ON instead of the first transistor, resulting in an erroneous read operation. The threshold voltage of a field effect transistor varies in a known way with its channel length, and the development of VLSI integration and miniaturization techniques has made the channel length shorter and shorter and the possible dispersion, or different, in the threshold voltages correspondingly larger and larger. The resulting increased possibility of sensing errors is a significant limitation upon higher integration and miniaturization.